The present disclosure relates to civil engineering, and more particularly to a method of semi-quantitatively evaluating concrete carbonation and an apparatus for semi-quantitatively evaluating concrete carbonation using the same.
In general, hardened concrete, as a hydration product of cement, exhibits strong basicity because a large amount of calcium hydroxide (Ca(OH)2; portlandite) is formed.
The calcium hydroxide is under the influence of carbon dioxide in the air from the surface of the concrete to slowly change into calcium carbonate (CaCO3; calcite) and water over time, and during this process, reaction products as illustrated in Table 1 are formed.
TABLE 1Concrete Carbonation productsPortland cement hydration productCarbonation productsCalcium hydroxideCalcite and waterCalcium silicate hydrateCalcite, silica gel and waterCalcium aluminate hydrateCalcite, alumina gel, and waterHydrated fertile phasesCalcite, ferric oxide, alumina gel,and waterEttringite and calciumGypsum, alumina gel, and watermonosulfoaluminate
Since calcite is neutral, the pH of the concrete gradually changes from strong alkaline to neutral depending on the degree to which portlandite is changed into calcite. The concrete is gradually carbonated from the surface into which carbon dioxide gas and water may easily penetrate.
This phenomenon is called as carbonation (neutralization) of concrete.
The carbonation phenomenon of concrete results in the corrosion of reinforcement steel in the concrete and the generation of swelling pressure, and thus, the carbonation phenomenon of concrete may cause degradation of durability of a structure, for example, cracks occur in the concrete due to the swelling pressure and degradation rapidly proceeds.
Thus, it is important to identify the carbonation of concrete in terms of the durability of the structure.
Typically, research to analyze the carbonation of concrete has been widely conducted, and typical methods of evaluating carbonation are as follows:
First, there is a method of using an indicator (phenolphthalein).
The indicator is used to identify a concentration of hydrogen ions by measuring acidic and basic levels of a measured material or determining the equivalence point of titration, at which acid and base are neutralized, based on the fact that the color of the indicator changes according to the concentration of hydrogen ions.
As described above, since concrete contains a large amount of portlandite in a hydrate, the concrete normally exhibits strong basicity (pH 12 to 13).
When carbon dioxide in the air acts on the concrete, calcite is formed to reduce portlandite exhibiting strong basicity in the hydrate, and thus, carbonation, which gradually reduces the pH of the concrete, proceeds.
Since the concentration of hydrogen ions in the concrete changes according to the progress of the carbonation, the progress of the carbonation may be determined by the presence of discoloration using the indicator.
Herein, it is general to use a phenolphthalein solution as the indicator.
Second, there is a method by differential thermal gravimetric analysis (TG-DTA).
This is a method of measuring thermal changes in the process of the release of bound water or absorbed water through thermal changes in which the energy absorbed or released when a crystal structure changes during the process of heating or cooling the concrete is converted to calories, wherein the method simultaneously performs differential thermal analysis (DTA) investigating an endothermic or exothermic reaction occurred due to the changes in temperature and thermogravimetry (TG) measuring weight change caused by temperature change.
The differential thermal gravimetric analysis (TG-DTA) is a method with high accuracy. However, since the carbonation proceeds from the surface to the inside of the concrete, the differential thermal gravimetric analysis has disadvantageous in that sampling must be conducted according to the depth of the concrete and test equipment is expensive. Thus, the differential thermal gravimetric analysis is used to complement the measurement results obtained by the indicator method.
Third, there is an X-ray diffraction (XRD) method.
The X-ray diffraction method is a method of analyzing a crystalline material, and in general, the objective thereof is to qualitatively analyze the amounts of portlandite and calcite.
An X-ray spectrometer is used to analyze a crystal structure of a material. When a crystal is irradiated with X-rays, X-rays are reflected from crystal lattice planes, and since a diffraction angle of a selected diffraction line and an intensity of X-rays are unique to each mineral, the X-ray diffraction method may distinguish types of the mineral.
In the method, similar to the differential thermal gravimetric analysis, concrete sampled according to the depth are crushed to be used as samples, and a carbonated portion is measured by comparing peak intensities of the portlandite and calcite.
Since the X-ray diffraction method, as a qualitative analysis method, is used to determine a relative amount of a component, there is a limitation in that the quantitative determination of the component is impossible only by the X-ray diffraction method itself. Thus, the X-ray diffraction method is used as an auxiliary means and is widely used for monitoring changes in results and relatively comparing detected components.
Therefore, in order to more accurately evaluate the carbonation of concrete, the analysis may be performed by using all of the above methods.
However, since this may be very cumbersome and there is no certain correlation between the results, there are difficulties in the evaluation.